1. Field of the Disclosure
The present invention relates generally to power supplies, and more specifically, the invention relates to power supply controllers.
2. Background
In general, a switched mode power supply controller regulates an output of the power supply by controlling the switching of a switch of the power supply. For instance, an example controller may regulate an output of the power supply by choosing whether or not to allow the switch to conduct current during each switching cycle. An enabled switching cycle is one in which the controller allows the switch to conduct. A disabled switching cycle is one in which the controller does not allow the switch to conduct. The controller produces a sequence of enabled and disabled switching cycles to regulate the output of the power supply.
In one type of controller, conduction of the switch terminates when the current in the switch reaches one of a plurality of current limit values. The controller chooses a particular current limit value in response to the sequence of past enabled and disabled cycles to avoid undesirable patterns in the sequence of enabled and disabled switching cycles. The undesirable patterns can generate audio noise from electrical and magnetic components. Also, such patterns can cause undesirable deviations in the regulated output, particularly when the regulated output is an output voltage.
The load of a power supply typically changes over a wide range. Each current limit value provides acceptable operation of the power supply over a limited range of load. The range of load associated with one current limit value overlaps the range of load assigned to an adjacent current value to assure smooth and stable operation over the entire range of loads. Therefore, two current limit values may provide acceptable operation for the same load.
The two current limit values correspond to two different modes of operation for the same load, each one with different conduction losses and different switching losses for the same load. The conduction losses and the switching losses reduce the efficiency of the power supply. Thus, the power supply may operate at different efficiencies for the same load, depending on which current limit value the controller selects for that load. Since such a controller may use a state machine, for example, to determine the current limit value from a sequence of enabled and disabled switching cycles, random changes in loading can produce different efficiencies for the same load.